1. Technical Field
The present invention relates generally to gas turbine engine turbine blade tip shrouds and, more particularly, to turbine blade tip shrouds for ceramic turbine blades.
2. Background Information
Some gas turbine engines employ blade tip shrouds on individual airfoils to limit blade amplitudes when vibrating due to forced or resonant response and to reduce aerodynamic flow losses and leakage over the tip of the airfoil and to guide fluid flow over the airfoils. This is particularly true in the low pressure section of a gas turbine engine. Neighboring shrouds abut in the circumferential direction to add mechanical stiffness, and provide damping during blade vibration. When a series of such assemblies are mounted together, the tip shrouds define, in effect, a continuous annular surface. Circumferentially opposite edges of the tip shrouds are provided with abutment faces and are designed to provide desired tip constraint at assembly and engine operating conditions.
Annular seal teeth extend radially outwardly from the shrouds to engage seal lands to seal the gas flowpath between the tip shrouds and casing surrounding the rotor. The seal lands typically are in the form of a honeycomb covered stator shroud. It is known to use ceramic or ceramic matrix composite (CMC) materials for turbine airfoils.
It is known that light weight, uncooled, high temperature capability, ceramic matrix composite (CMC) airfoils may be used for turbine blades in the low pressure turbine. Ceramic and ceramic matrix composite (CMC) materials are low strain to failure materials. One ceramic matrix composite material suitable for turbine blades is a SiC—SiC CMC, a silicon infiltrated silicon carbide composite reinforced with coated silicon carbide fibers. CMC's are an attractive alternate material to Nickel based superalloy low pressure (LPT) blades because of their high temperature capability and light weight. These characteristics provide opportunities for cooling flow savings as compared to cooled LPT blades. This also provides possible improvement in design optimization of disks which support LPT blades.
Design challenges posed by CMC LPT blades include low thermal coefficient of expansion, low strain to failure, and relatively poor wear characteristics. The low thermal coefficient of expansion results in smaller growth of the tip shroud in the tangential direction during operation relative to metal blades. The impact of this is a reduction and possible loss of interlock load between adjacent blade tip shrouds which may also be a potential HCF issue and an increase in leakage area around the tip shrouds perimeter which is a performance issue.
Due to the brittle nature and lack of damage tolerance of CMC's compared to metals, the material is very susceptible to chipping, cracking, and impact damage. For these reasons, CMC on CMC contact at the interlock faces of the blade tip shrouds is a design concern. The main concern is loss of material and reduction and possible loss of interlock load. The poor wear characteristics are an issue with regards to the rotating seal teeth cutting the static shroud honeycomb and the relative motion of interlock surfaces on adjacent LPT blade tip shrouds.
The impact of wear on the seal teeth can increase environmental degradation resulting in an excessively large leakage path between the seal teeth and the shroud which lowers overall engine performance and fuel efficiency. It can also result in fraying of CMC plies which is a durability issue. The impact of excessive wear on the interlock surface could be a loss of interlock load resulting in an undamped airfoil prone to forced or resonant response.
Accordingly, it is desirable to have a CMC LPT blade and blade tip shroud designs which lower or prevent loss of CMC material and prevent reduction and possible loss of interlock load between adjacent blade tip shrouds, and possible sealing issues.